{"record":{"id":"841865e32d03b210","repo":"dotnet/machinelearning","slug":"strings-badcolumncast-formatted-with-column-datat-841865","errorCode":null,"errorMessage":"Strings.BadColumnCast (formatted with column.DataType, typeof(Decimal))","messagePattern":"Strings\\.BadColumnCast \\(formatted with column\\.DataType, typeof\\(Decimal\\)\\)","errorType":"exception","errorClass":"ArgumentException","httpStatus":null,"severity":"error","filePath":"src/Microsoft.Data.Analysis/DataFrameColumnCollection.cs","lineNumber":359,"sourceCode":"\n            throw new ArgumentException(string.Format(Strings.BadColumnCast, column.DataType, typeof(Double)));\n        }\n\n        /// <summary>\n        /// Gets the <see cref=\"DecimalDataFrameColumn\"/> with the specified <paramref name=\"name\"/>.\n        /// </summary>\n        /// <param name=\"name\">The name of the column</param>\n        /// <returns><see cref=\"DecimalDataFrameColumn\"/>.</returns>\n        /// <exception cref=\"ArgumentException\">A column named <paramref name=\"name\"/> cannot be found, or if the column's type doesn't match.</exception>\n        public DecimalDataFrameColumn GetDecimalColumn(string name)\n        {\n            DataFrameColumn column = this[name];\n            if (column is DecimalDataFrameColumn ret)\n            {\n                return ret;\n            }\n\n            throw new ArgumentException(string.Format(Strings.BadColumnCast, column.DataType, typeof(Decimal)));\n        }\n\n        /// <summary>\n        /// Gets the <see cref=\"SingleDataFrameColumn\"/> with the specified <paramref name=\"name\"/>.\n        /// </summary>\n        /// <param name=\"name\">The name of the column</param>\n        /// <returns><see cref=\"SingleDataFrameColumn\"/>.</returns>\n        /// <exception cref=\"ArgumentException\">A column named <paramref name=\"name\"/> cannot be found, or if the column's type doesn't match.</exception>\n        public SingleDataFrameColumn GetSingleColumn(string name)\n        {\n            DataFrameColumn column = this[name];\n            if (column is SingleDataFrameColumn ret)\n            {\n                return ret;\n            }\n\n            throw new ArgumentException(string.Format(Strings.BadColumnCast, column.DataType, typeof(Single)));\n        }","sourceCodeStart":341,"sourceCodeEnd":377,"githubUrl":"https://github.com/dotnet/machinelearning/blob/7b76e69cf964daeca3f1377af6bc5543284d56c6/src/Microsoft.Data.Analysis/DataFrameColumnCollection.cs#L341-L377","documentation":"DataFrameColumnCollection.GetDecimalColumn(name) only returns a column when it is exactly a DecimalDataFrameColumn. When a column exists under that name but has a different data type, the library throws ArgumentException with Strings.BadColumnCast, formatted with the actual column DataType and typeof(Decimal). It signals a type mismatch between the requested accessor and the stored column, not a missing column.","triggerScenarios":"Calling df.Columns.GetDecimalColumn(\"name\") on a DataFrame whose column \"name\" exists but is not a DecimalDataFrameColumn (e.g. it was loaded as double, float, int, or string). The indexer this[name] succeeds, the `is DecimalDataFrameColumn` pattern fails, and the ArgumentException at src/Microsoft.Data.Analysis/DataFrameColumnCollection.cs:359 is thrown.","commonSituations":"Financial data loaded as double by CSV inference; database DECIMAL columns mapped to double or string during load; schema drift after a data source change; hardcoded column names pointing at a differently-typed column.","solutions":["Check the column's actual type first: df.Columns[\"name\"].DataType (or the column class name) and use the matching Get<ColumnType>Column accessor.","Convert the column to decimal before access: df.Columns[\"name\"] = df.Columns[\"name\"].Cast<decimal>(), then call GetDecimalColumn.","If the type can vary, access generically via df[\"name\"] and inspect/convert values instead of using the typed accessor.","Fix upstream data ingestion so the column is loaded as decimal."],"exampleFix":"// before\nvar col = df.Columns.GetDecimalColumn(\"Amount\"); // throws if \"Amount\" is double\n// after\nif (df.Columns[\"Amount\"].DataType == typeof(decimal))\n{\n    var col = df.Columns.GetDecimalColumn(\"Amount\");\n}\nelse\n{\n    df.Columns[\"Amount\"] = df.Columns[\"Amount\"].Cast<decimal>();\n    var col = df.Columns.GetDecimalColumn(\"Amount\");\n}","handlingStrategy":"type-guard","validationCode":"// before calling GetDecimalColumn\nif (df.Columns.Contains(\"name\") && df.Columns[\"name\"].DataType != typeof(decimal))\n    throw new InvalidOperationException($\"Column 'name' is {df.Columns[\"name\"].DataType}, expected decimal\");","typeGuard":"static bool IsDecimalColumn(DataFrameColumn c) => c is DecimalDataFrameColumn;\n// usage: if (df.Columns[\"name\"] is DecimalDataFrameColumn decCol) { ... }","tryCatchPattern":"try\n{\n    var col = df.Columns.GetDecimalColumn(\"name\");\n}\ncatch (ArgumentException ex) when (ex.Message.Contains(\"column\"))\n{\n    // inspect df.Columns[\"name\"].DataType, convert with .Cast<decimal>(), and retry\n}","preventionTips":["Always check df.Columns[name].DataType before using a typed Get*Column accessor.","Prefer generic access via df[\"name\"] plus explicit .Cast<decimal>() when column types come from external data.","Financial data loaded via CSV inference usually becomes double; convert to decimal explicitly.","Log the loaded schema after reading external data to catch inference surprises early."],"tags":["dotnet","dataframe","type-mismatch","argument-exception"],"backgroundTag":"type-mismatch","analyzedSha":"7b76e69cf964daeca3f1377af6bc5543284d56c6","analyzedAt":"2026-09-11T12:35:38.930Z","contentChangedAt":"2026-09-11T12:35:38.930Z","schemaVersion":2},"datasetVersion":"2026-09-14T05:17:10.506Z"}